A quiet little spot where Rod Mollise shares his adventures and misadventures...

Sunday, March 26, 2017

Issue #536: Deep Sky Imaging in Seven Easy (Sorta) Steps

All those cables!

We’ve spent the last several weeks setting you up with a
telescope, mount, camera, and guide system. Now it is finally time to get outside with all
that gear (assuming you, unlike me, have clear skies) and use it to capture the
deep sky wonders of Spring.

Step One: Set Up

Naturally you’ve got to assemble the telescope and mount. But
where should you assemble them? Unless your backyard is really, really horrible
light pollution wise, I strongly council you to use the good, old back forty
the first couple of times you work with the new gear. You’ll be dealing with a
bunch of unfamiliar equipment and some complex tasks, and it’s always easier to
do that at home where you can run inside for a look at a manual (or for a quick
drink if you get really stressed) under white light rather than squinting at
the instructions with a red flashlight at a dark site.

Anyhow, set up tripod and mount and level them. How precise
does level need to be for a German equatorial? Technically, you don’t have to
level the mount at all. All it needs
is to be level enough so that it doesn’t tip over. In some cases, being close to level can make
polar alignment easier, however. Level won’t affect polar alignment, period, but
being near level means the mount’s altitude and azimuth adjusters won’t
interact. When you move in altitude, it doesn’t also affect azimuth, and
vice-versa, making it quicker to dial in alignment.

Next, attach the counterweight(s) to the declination shaft. Always mount the weights first, followed by
the telescope. You will be mighty unhappy if you do the reverse, your R.A.
lock isn’t secure, and the telescope slams into the tripod. When tearing down
at the end of the run, reverse that. Remove the telescope first, then the
weights. Once the scope is safely on the mount, install everything you’ll be
placing on the tube: guide-scope and
camera, imaging camera, finders, etc.

Balance is very important if an inexpensive GEM is to track
well, so spend enough time with that to get it exactly right. First, decide
which side of the Meridian you’ll be imaging on, east or west, and balance
accordingly. You’ll always want to be slightly east heavy. If you are going to
be imaging on the West side of the Meridian, you should be “scope heavy.” On
the east side? “Counterweight heavy.”

Polar alignment with Sharpcap...

Balance in R.A. first. Point the scope north, lock the
declination lock and, with the counterweight down and halfway up the shaft (if
you don’t know approximately where it should be), undo the R.A. lock at least
partway and move the mount in R.A. so the counterweight bar is level. Do not
let go of the scope. Now, still without completely letting go, allow the scope
or weight to rise or sink. When you’ve determined which way the weight needs to
go on the shaft, up or down, return the mount to counterweight down position,
lock the R.A. lock and move the counterweight as required. Return the mount to
counterweight bar level, and see if balance is perfect. Keep on going with this
procedure until it is.

Now for the East heavy bit. When you are in perfect balance,
move the weight about ½-inch up the shaft if you are imaging to the west, and
½-inch down the shaft if you are imaging to the east. That should be more than
enough to ensure the R.A. gears remain
constantly meshed in the interests of good tracking.

For declination balance, return the mount to the
counterweight bar level position, lock the R.A. lock and, holding on to the
telescope, release the declination lock. When you know which way the scope
needs to go in the mount saddle, forward or back, return to the counterweight
down position, move the scope (carefully) as required return to the
counterweight bar level position, and check. Keep going till the scope is
balanced in declination.

What if your mount, like many in this class, is a little
stiff on the declination axis and is somewhat difficult to balance? Don’t worry
about it too much. Your mount is not tracking in declination, and if you’ve
done polar alignment well, PHD shouldn’t have to issue many guide corrections
on that axis. Good R.A. balance is far more important; declination balance can
be “approximate” without hurting anything.

Step Two: Polar Alignment

If you are using a polar alignment borescope, Polemaster,
Sharpcap, or the Kochab’s Clock method, do polar alignment now. None of these
methods require the scope to be powered up and tracking, so it’s convenient to
do the polar alignment before the mount is all festooned with cables and hand
controls (I use Sharpcap these days). Take your time and do as good a polar
alignment as you possibly can; you’ll thank me later.

Step Three: Hooking Up

Connecting to the mount...

Plug in all the cables and the telescope’s hand control. You’ll
have at least four and maybe five cords to deal with: Power cable, serial cable for scope control,
imaging camera USB connection, guide camera USB connection, and an ST-4 cable
if you’re going to guide through the mount’s auto-guide port. Try to do a neat
job with the cords, arranging them so they are not prone to snagging on the
mount or tripod—which will ruin your guiding. Don’t forget to attach dew heater
strips, dew heater controller, and dew shield if you need them in your
environment.

Step Four: Goto Alignment

Take care of goto alignment now. Do whatever procedure is
required to get the mount going to its gotos. How exactly do you line up the
alignment stars, though? You can choose one of two methods. You can either
remove the camera from the telescope and temporarily replace it with diagonal
and eyepiece, or you can use the camera to do the alignment. In the beginning,
it may be easier to remove the camera and do the goto alignment with an
eyepiece.

If you do use the camera, you’ll, of course, need to turn it on (and
the laptop and its software if you are tethering to a PC), and center the alignment stars on the
camera’s display or the laptop screen. Since alignment stars are bright, one
will allow you to get rough focus, too.

If you choose to use Celestron’s AllStar Polar Alignment, which
is built into the hand control firmware, take care of that once goto alignment
is done—ASPA requires the goto alignment be accomplished first. If you do a declination
drift polar alignment (horrors), now is also the time to do that, since having
the telescope tracking during the procedure makes things much easier and is
practically required.

Step Five: Focus

The Double Cluster is an easy and pretty target...

If you goto aligned using the DSLR, you’ve got focus roughed
in, and can now do a fine focus procedure. If you used an eyepiece instead of
the camera, however, get rough focus with the imaging camera at this time. If
the last alignment star was a good, bright one, stay on it and use it to focus.

To get in the focus ballpark, adjust the focuser on the telescope (I
am a big fan of remote moto-focus for imaging) until the bright star is as
small as you can make it and dimmer field stars begin to appear and sharpen.
Exposure? I like one to two seconds; that allows me to see results quickly
after tweaking focus. Set camera ISO as high as needed to get a good image of
the stars. If you are way out of focus, you may need to max ISO out and
increase exposure time till you detect the big round globe of a star (in a
refractor). Once it is closer to focus, back off on ISO and exposure for a less
overexposed star image.

When the field stars are as small as possible by eye, tweak
focus with a fine-focus method of choice, which may be a Bahtinov focus mask, or a focusing routine built
into imaging software (like Nebulosity) if you are tethering the camera.

Step Six: Acquire Target and Compose Shot

Rough focus done, I interface my planetarium program
(Stellarium these says) to the GEM. I start Stellarium (or whatever), and
connect it to the telescope mount, that is. How? I invariably use the ASCOM
telescope driver system, even if the program on the laptop, like Stellarium,
has built-in telescope drivers. Why? Because ASCOM includes a little onscreen
telescope HC that allows me to move the mount (at different rates) from the computer.
That means I don’t have to get up and walk out to the scope and HC, press a
direction button to center the object, walk back to the computer, etc.

A Bahtinov Mask makes fine focusing easy...

Alright, time to get on our first subject. What should that
be? Even if you are at least somewhat experienced in deep sky imaging, begin
with something easy with this new rig. This time of year, perhaps a nice winter
open cluster over in the west like M35 or M37 or the Double Cluster. One
important consideration given the economical mounts we’re using? Stay away from the Meridian. These GEMs
just don’t track well in that area. Don’t image an object that will come within
10-degrees of the Meridian before your sequence is done, and don’t begin
imaging an object until it is at least 10-degrees past the Meridian.

Once the scope goto stops, take an exposure long enough to reveal your target to see how
the composition of the shot looks. If the subject is not centered, or just not
framed the way I want it, I use the ASCOM HC to fix that. I keep the exposures short
enough to make framing easy, maybe referencing a bright star in the frame if
the object doesn’t quite show up in 1 – 3-second shots.

What if the target object is not in the field of the camera at all when the goto slew is done? That’s not much of a concern these days for most mounts,
but if you have a problem, a quick solution is to slew to a nearby bright star,
center it with the aid of your finder and “sync” on it. You should then be able
to slew back to the target and have it in the frame. Oh, before you do that, be
sure it really isn’t in the frame. If
the target is a dimmer one, increase exposure and ISO and see if it appears.

When the subject is properly centered fire up PHD2 and get
auto-guiding going. The main gotchas there? Make sure the guide scope is
well-focused and that the guide star you’ve chosen is neither too dim nor too
bright (saturated). When you put the cursor on a star, PHD2 will tell you all
about that. Some imagers believe the guide star should be slightly out of focus
for best guiding, but I’ve found I get better results from sharp stars.

When the mount is guiding, I go back to the imaging camera
and do a test exposure. How long should that test subframe be? That depends on
the sky and the subject. If I’m in the backyard, going much beyond a minute
causes the background sky to brighten up so much that processing can be
difficult later. At my dark site, I’ll expose for 2 – 5-minutes. Exposure also
depends on the subject. An open cluster like M37 will be just fine in 30-second
– 1-minute subs. The Horsehead Nebula will not be.

One important thing to remember is that while you’ll be
stacking many shorter sub-frames into a finished exposure, you still have to
have each individual exposure long enough to pick up all the detail you need.
Stacking subframes will make the final result smoother and less noisy, but will not show any detail not present in
the individual subframes. Longer subframes are always better.

Get your guiding going on...

What should camera ISO (nee “ASA”) be set to? Normally, use
as low a value as you can to capture the detail you want while keeping noise
down. The higher the ISO, the noisier the image will be (and the brighter a
light polluted background). I rarely go above ISO 1600, and try not to exceed
800 in the backyard if possible. Naturally, ISO and exposure time interact. In
general, I’ve found it better to go with a lower ISO and a longer exposure when
possible.

Also examine the test exposure for signs of star trailing.
Assuming PHD is not going wacky on you, you’ve got a good polar alignment, and
the seeing is OK, that should not be a problem at the 400 – 600mm focal lengths
we’re using. If the stars don’t look right, go back to PHD and make sure it’s
still guiding well (bring up the graph as shown in the image here). If it
isn’t, you’ll have to troubleshoot.

If the stars are eggs or worse, first make sure the values
you’ve entered for the guiding parameters are close to those we outlined here. One variable that can change
from night to night is the guide camera’s exposure. While mounts in this class
tend to do best with 1 – 1.5-second guide exposures, if seeing is not good a
somewhat longer one can improve guiding. There’ll be less tendency for PHD to
try to guide on movement caused by seeing.

Step Seven: Expose

Time to do what we came for, take an exposure sequence. Set the
laptop program or the intervalometer to take a number of subframes at the
exposure value you determined was best. How many? As many as possible. Even on
an easy object like M37, more subs always make for a better looking finished picture.
I generally aim for 20 - 30.

Before beginning the sequence, though, let’s put the dark
frame question to bed. Since a DSLR’s sensor chip is not cooled, dark frames
are mandatory to eliminate the false stars of thermal noise. There are two ways
to subtract dark frames from subs, manually or automatically.

If you go manual, finish the imaging run and then, just
before packing up for the night, cover the telescope objective and shoot
subframes equal in number to the maximum number of lights you’ve taken in a
sequence. For example, if you did one 20 and one 30-subframe sequence, take
30-darks. If you used different exposures on different sequences, you’ll have
to do more than one sequence of darks—dark frame exposure values need to be the
same as their corresponding lights. The darks will be subtracted from the light
frames during image processing. This way of working is certainly acceptable,
but, in addition to being labor intensive, it isn’t as effective as it could be
in my opinion.

Me, I go automatic on dark frames for a couple of reasons.
Not only do I not have to worry about messing with darks at the end of the
evening or during processing, I think automatic darks are more effective.

How do you do auto dark frames? DSLRs allow you to select a
mode called “long exposure noise reduction” (or a similarly named menu item).
Engage that, and the camera will take a dark frame after each exposure and
automatically subtract it. Yes, that means an imaging run will take twice as
long as it otherwise would—30-minutes of subs will take an hour to complete—but
I think the results are just better.

Expose!

Why would the results be improved by taking a dark after
each light? Because the temperature of the DSLR’s sensor chip will vary
throughout the night. Ambient temperature will drop throughout the evening, and,
as an exposure sequence goes on, the internal temperature of the camera due to its
electronics will tend to rise. To be most effective, a dark should be taken as
soon after its matching light subframe as possible, so the temperature it was
exposed at is close to that of the light frame.

OK, set that computer or intervalometer for the number of
exposures at the required exposure value, push the “go” button and…and…wander
around and do something else while the exposure sequence completes. I usually just
go inside and watch TV. If I’m at the dark site, I’ll cadge looks through my
buddies’ telescopes. I’ll come back periodically and see how things are
going—especially how PHD is guiding—but I rarely encounter problems unless
clouds have moved in and my guide star has been lost, temporarily or
permanently.

Once the sequence is finished, it’s time to go on to the
next target. How many targets should you do? That’s for you to decide, but I
tend to believe fewer targets, maybe just one or two per evening, and more
subframes (and maybe longer exposures) is the way to go.

Done, I’ll pack everything up and head for home if I’m at
the dark site, or, if I’m in my secure backyard, I’ll just cover the refractor and GEM with
my Telegizmos cover and only take the computer inside—which is a much more
pleasant way to end an evening under the stars than having to disassemble
everything and carry it back into the house when I’m tired.

And next? Next is processing the images, but that is a story
for some other Sunday.

In step 6, you mention using the ASCOM telescope/mount driver for the benefit of the on-screen hand control. There's second important reason to avoid using the proprietary telescope interface of planetarium programs. The ASCOM drivers can share the communication with the mount for things like guiding, and information gathering. For example, some imaging software can create FITS files with embedded imaging coordinates collected from the mount. (Note: Some ASCOM drivers require the ASCOM hub, POTH. It's free and included. But other drivers (eg, Gemini, Celestron) have built-in sharing of the mount.)

But I must note that Nebulosity, the program used for capture here, has a bug* with certain Canon cameras (like my SL1) that does not allow exposures longer than 1 minute if the "long exposure noise reduction" option is enabled. That at least in the Mac version (I have not been able to verify it yet in the Win version).

*Basically, Nebulosity does not wait enough time for the camera to finish its work and when it does not receive an image it generates an error.

Nice series of "how to" AP articles for "idiots" like me. To follow up on Joe's question, did I hear somewhere that you will HAVE to use ASCOM if you want to use Stellarium? Something about Stellarium's native drivers not allowing any other communication with other functions (guiding, etc.). Seems like most of the serious imagers go ASCOM anyway. Just curious, as I probably won't use Stellarium for control with my AVX and AP sessions. It's not like I'm going to flit from object to object when taking pictures.

The built in drivers work fine if all you want to do is send the mount on gotos and guide using ST-4 mode. Or you can use the built in drivers and use PHD (or whatever) connected through ASCOM or you can run everything through ASCOM. ;)

I find Stellarium very nice to use even for one or two targets a night. It makes composing shots and fine tuning aim much easier than with just the AVX hand control. ;)